The present invention relates to ferrite magnet powder, magnet made from the magnet powder and methods for making the powder and the magnet.
Ferrite is a generic term for various compounds containing an oxide of a divalent anionic metal and trivalent iron and has found a wide variety of applications in motors, electric generators, and so on. Typical materials for a ferrite magnet include Sr or Ba ferrites with a magnetoplumbite hexagonal structure (SrFe12O19 or BaFe12O19). Each of these ferrites can be made from iron oxide and a carbonate of strontium (Sr) or barium (Ba) at a relatively low cost by a powder metallurgical process.
A basic composition of a magnetoplumbite ferrite is usually represented by a chemical formula of MO.nFe2O3, where M is a metal element to be divalent anions and is selected from the group consisting of Sr, Ba, Pb and so on. In the ferrite, iron ions (Fe3+) located at respective sites have spin magnetic moments and are bonded together by superexchange interaction with oxygen ions (Oxe2x88x922) interposed therebetween. At each of these sites, Fe3+ has an xe2x80x9cupwardxe2x80x9dor xe2x80x9cdownwardxe2x80x9d magnetic moment with respect to the c-axis. Since the number of sites with the xe2x80x9cupwardxe2x80x9d magnetic moment is different from that of sites with the xe2x80x9cdownwardxe2x80x9d magnetic moment, the, ferrite crystal exhibits ferromagnetism as a whole (and is, called a xe2x80x9cferrimagnetismxe2x80x9d).
It is known that the remanence Br, which is one of the indices representing the magnetic properties of a magnetoplumbite ferrite magnet, can be improved by enhancing Is of the crystal or increasing the density of a ferrite sintered compact and aligning the orientations of the crystal more fully. It is also known that the coercivity HCJ of a ferrite magnet can be enhanced by increasing the fraction of single domain crystals existing in the magnet. However, if the density of the sintered compact is increased to improve the remanence Br, then the ferrite crystals grow at a higher rate, thus decreasing the coercivity HCJ. Conversely, if the grain sizes are controlled with the addition of Al2O3, for example, to increase the coercivity, then the density of the sintered compact decreases, resulting in decrease of the remanence. Compositions, additives and production conditions for ferrites have been researched and developed from various angles to improve these magnetic properties of a ferrite magnet. However, it has been difficult to develop a ferrite magnet with its remanence and coercivity both improved.
The present applicant developed a ferrite magnet with the coercivity improved by adding Co to the source material and without decreasing the remanence thereof (see Japanese Patent Gazette for Opposition Nos. 4-40843 and 5-42128).
After that, a ferrite magnet with the saturation magnetization "sgr"s improved by replacing Fe and Sr with Zn and La, respectively, was proposed (see Japanese Laid-Open Publication Nos. 9-115715 and 10-149910). As described above, a ferrite magnet is a ferrimagnetism in which the magnetic moments at respective sites of Fe3+ are in the opposite directions, and therefore has relatively low saturation magnetization. However, according to the above-identified laid-open publications, if ions with a smaller magnet moment than that of Fe are placed at particular sites of Fe ions, then the number of sites with the xe2x80x9cdownwardxe2x80x9d magnet moment will decrease and the saturation magnetization "sgr"s will increase. Examples of using Nd or Pr instead of La and using Mn, Co or Ni instead of Zn are also described in these publications.
A ferrite magnet with a composition Sr1xe2x88x92xLaXCoxFe12xe2x88x92xO19, which additionally contains La and Co to increase both the coercivity HCJ and saturation magnetization "sgr"s thereof, is disclosed in xe2x80x9cDigests of the 22th Annual Conference on Magnetics in Japanxe2x80x9d (which was distributed on Sep. 20, 1998).
However, even these ferrite magnets cannot exhibit sufficiently improved coercivity and saturation magnetization. In particular, if the Sr1xe2x88x92xLaxCoXFe12xe2x88x92xO19 compound, in which Fe and Sr are replaced with Co and La, respectively, is calcined at such a temperature as that disclosed in Japanese Laid-Open Publication No. 10-149910 (i.e., 1200xc2x0 C.), then the resultant coercivity is not high enough although rather high saturation magnetization "sgr"s is attainable.
An article included in xe2x80x9cDigests of the 22th Annual Conference on Magnetics in Japanxe2x80x9d (which was distributed on Sep. 20, 1998) reports that coercivity would be increased to a certain degree by replacing Fe with Co, not Zn. However, the article does not specify the causes. In addition, neither coercivity nor remanence seems to be sufficiently improvable.
In view of these respects, a primary object of the present invention is to provide a ferrite magnet powder with saturation magnetization and coercivity both improved and a magnet made from the magnet powder.
An inventive magnet powder has a ferrite primary phase represented as (1xe2x88x92x)SrO.(x/2)La2O3.(nxe2x88x92y/2)Fe2O3.yCoO, where x and y represent mole fractions and 0.1xe2x89xa6xxe2x89xa60.4, 0.1xe2x89xa6yxe2x89xa60.4 and 5.5xe2x89xa6nxe2x89xa66.5. Fe has a magnetic moment oriented upwardly with respect to a crystal c-axis at a number of sites thereof, and also has an opposite magnetic moment oriented downwardly with respect to the crystal c-axis at another number of sites thereof. And Fe is replaced with Co at the greater number of sites thereof.
The magnet powder is preferably calcined at a temperature higher than 1300xc2x0 C.
In a preferred embodiment, the magnet powder shows a magnetic anisotropy field HA of 1750 kA/m (22 kOe) or more and a saturation magnetization "sgr"s of 84.78 xcexcWbm/kg (67.5 emu/g) or more at room temperature.
An inventive bonded magnet is characterized by containing the magnet powder. On the other hand, an inventive sintered magnet is characterized by being made from the magnet powder.
An inventive method for making a magnet powder includes the steps of: preparing a source material blended powder, in which oxide powders of La and Co are added to powders of SrCo3 and Fe2O3, respectively; calcining the source material blended powder at a temperature higher than 1300xc2x0 C. and equal to or lower than 1450xc2x0 C., thereby forming a ferrite calcine with the composition of (1xe2x88x92x)SrO.(x/2)La2O3.(nxe2x88x92y/2)Fe2O3.yCoO, where 0.1xe2x89xa6xxe2x89xa60.4, 0.1xe2x89xa6yxe2x89xa60.4 and 5.5xe2x89xa6nxe2x89xa66.5; and pulverizing the calcine. Note that the step of preparing the source material blended powder refers to not only making such a source material blended powder from the beginning, but also purchasing and using a source material blended powder that was made by others and blending powders prepared by others.
The calcining step is preferably carried out at a temperature equal to or higher than 1350xc2x0 C.
An inventive method for producing a magnet includes the steps of: preparing a source material blended powder, in which oxide powders of La and Co are added to powders of SrCo3 and Fe2O3, respectively; calcining the source material blended powder at a temperature higher than 1300xc2x0 C. and equal to or lower than 1450xc2x0 C., thereby forming a ferrite calcine with the composition of (1xe2x88x92x)SrO.(x/2)La2O3.(nxe2x88x92y/2)Fe2O3. yCoO, where 0.1xe2x89xa6xxe2x89xa60.4, 0.1xe2x89xa6yxe2x89xa60.4 and 5.5xe2x89xa6nxe2x89xa66.5; pulverizing the calcine to obtain a ferrite magnet powder; and, shaping and sintering the ferrite magnet powder.
Another inventive method for producing a magnet includes the steps of: preparing a source material blended powder, in which oxide powders of La and Co are added to powders of SrCo3 and Fe2O3, respectively; calcining the source material blended powder at a temperature higher than 1300xc2x0 C. and equal to or lower than 1450xc2x0 C., thereby forming a ferrite calcine with the composition of (1xe2x88x92x)SrO.(x/2)La2O3.(nxe2x88x92y/2)Fe2O3.yCoO, where 0.1xe2x89xa6xxe2x89xa60.4, 0.1xe2x89xa6yxe2x89xa60.4 and 5.5xe2x89xa6nxe2x89xa66.5; pulverizing the calcine to obtain a ferrite magnet powder; and forming a bonded magnet from the ferrite magnet powder.
The calcining step is preferably carried out at a temperature equal to or higher than 1350xc2x0 C. Another inventive magnet powder has a ferrite primary phase represented as (1xe2x88x92x)A0(x/2)R2O3.(nxe2x88x92y/2)Fe2O3.yCoO, where A is at least one element selected from the group consisting of Sr. Ba, Ca and Pb; R includes at least one element selected from the group consisting of rare-earth elements including Y and Bi; x and y represent mole fractions; and 0.1xe2x89xa6xxe2x89xa60.4, 0.1xe2x89xa6yxe2x89xa60.4 and 5.5xe2x89xa6nxe2x89xa66.5. Fe has a magnetic moment orient upwardly with respect to a crystal c-axis at a number of sites thereof, and also has an opposite magnetic moment oriented downwardly with respect to the crystal c-axis at another number of sites thereof. And Fe is replaced with Co at the greater number of sites thereof.
The magnet powder is preferably calcined at a temperature higher than 1300xc2x0 C.
Still another inventive magnet is characterized by being made from the magnet powder.